DEEP-SEA SEDIMENTS RECORD THE GEOPHYSIOLOGY OF THE LATE ORDOVICIAN GLACIATION

Sedimentological analysis of the Ordovician-Silurian international bou ndary stratotype section at Dob's Linn, Scotland shows systematic chan ges in muddy turbidite microfacies which are interpreted in terms of a four phase glaciation. This comprised: (1) a, pre-glacial cooling of the deep sea in the early Ashgill (Cautleyan), (2) initiation of inten se thermohaline circulation in the late Rawtheyan, predating, (3) the start of the glacial maximum near the end of the Rawtheyan, and (4) ra pid de-glaciation in the mid- to upper Hirnantian (lower part of the p ersculptus Biozone). It is proposed that the glacial maximum was trigg ered by a combination of negative greenhouse effect ('Monterey Hypothe sis') and increasing surface albedo. Cessation of thermohaline circula tion and the expansion of anoxia mark the rapid end of the glaciation in this deep-sea environment.